The present invention relates generally to semiconductor fabrication and, more particularly, to a system and method for determining an endpoint in the chemical mechanical planarization of thin films, particularly metal films comprised of copper.
In the fabrication of semiconductor devices, chemical mechanical planarization (CMP) is used to planarize globally the surface of an entire semiconductor wafer. CMP is often used to planarize metallization layers, which are formed of conducting metals, e.g., aluminum and copper. To obtain consistent results as well as to avoid damaging the underlying circuit components, the endpoint of the CMP operation must be carefully monitored.
A variety of approaches have been used to detect an endpoint in the CMP of metallization layers. Direct methods of determining an endpoint use an external signal source or a chemical agent to determine the physical state of the wafer surface during the CMP operation. In such direct methods, the wafer surface has been monitored using acoustic wave velocity, optical reflectance and interference, impedance/conductance, and electrochemical potential change due to the introduction of specific chemical agents.
Indirect methods of determining an endpoint monitor a signal that is internally generated within the tool due to physical or chemical changes that occur naturally during the CMP operation. In such indirect methods, the following parameters have been monitored: the temperature of the polishing pad/wafer surface; the vibration of the planarization tool; the frictional forces between the polishing pad and the polishing head; the electrochemical potential of the slurry; and acoustic emissions.
The indirect methods of determining an endpoint are strongly dependent on the process parameters and the selection of consumables. Consequently, with the exception of friction sensing, none of the indirect methods has been widely used in the industry. Of the direct methods of determining an endpoint, a number of the optical methods have been used in the industry. One drawback of these optical methods, however, is that they do not provide a global indication of the state of the removal of the metal from the surface of the wafer. Instead, these optical methods detect the removal of metal from only a single localized area of the wafer.
In view of the foregoing, there is a need for a method that can reliably determine the endpoint in a CMP operation based on the state of the film removal over the entire surface of the wafer.
Broadly speaking, the present invention fills this need by providing a method for determining the endpoint in a chemical mechanical planarization (CMP) operation based on the concentration of an oxidizing agent in the slurry byproduct. The present invention also provides a CMP system configured to implement the method for determining an endpoint.
In accordance with one aspect of the present invention, a method for determining an endpoint in a CMP operation is provided. In this method, a concentration of an oxidizing agent in a slurry byproduct generated during a CMP operation is monitored. The endpoint of the CMP operation is determined based on the concentration of the oxidizing agent in the slurry byproduct. In one embodiment, when the concentration of the oxidizing agent in the slurry byproduct increases to a predetermined level, the CMP operation is stopped.
In one embodiment, the CMP operation is conducted on a metal film comprised of copper, and the oxidizing agent is comprised of hydrogen peroxide. In other embodiments, different oxidizing agents are used. By way of example, the oxidizing agent may be comprised of a material selected from the group consisting of HCl, nitric acid, hydroxylamine, KMnO4, and KIO3.
In one embodiment, the operation of monitoring the concentration of the oxidizing agent in the slurry byproduct includes diverting the slurry byproduct from a surface of a polishing pad, and measuring an optical property of the slurry byproduct diverted from the surface of the polishing pad. In one embodiment, the optical property of the slurry byproduct is measured with a refractometer. In one embodiment, the slurry byproduct is diverted from the surface of the polishing pad by a slurry diverter that is disposed downstream of a polishing head by a distance in a range from about 3 inches to about 5 inches. As used herein, the term xe2x80x9caboutxe2x80x9d means that the parameter specified can be varied within an acceptable manufacturing tolerance, e.g. , xc2x110%.
In accordance with another aspect of the present invention, a CMP system is provided. The CMP system may be either a linear CMP system or a rotary CMP system. In the case of a linear CMP system, the CMP system includes a pair of drums. Each of the pair of drums is configured to rotate. A polishing pad is disposed around the pair of drums. A polishing head, which is configured to hold a semiconductor wafer, is disposed above a top surface of the polishing pad. The CMP system further includes a slurry dispenser, e.g. a slurry bar, for dispensing a slurry onto the top surface of the polishing pad and a slurry diverter for diverting a slurry byproduct from the top surface of the polishing pad. A slurry catcher for receiving the slurry byproduct diverted from the top surface of the polishing pad is disposed adjacent to the polishing pad. An optical measuring tool for measuring an optical property of the slurry byproduct also is provided.
In the case of a rotary CMP system, the polishing pad is disposed on a tabletop that is configured to rotate, and the slurry dispenser may be a single nozzle of a flow of slurry emanating from the middle of the tabletop. The slurry diverter, the slurry catcher, and the optical measuring tool may be configured relative to the polishing pad in substantially the same manner described for a linear CMP system.
In one embodiment, the optical measuring tool is a refractometer. In one embodiment, the distance the slurry diverter is disposed away from the polishing head is in a range from about 3 inches to about 5 inches. In one embodiment, the slurry catcher is comprised of a substantially transparent material, and the optical measuring tool is disposed below the slurry catcher.
The CMP system and method of the present invention advantageously enable the endpoint of a CMP operation to be determined by monitoring the concentration of an oxidizing agent in the slurry byproduct. The method is well suited for use in the CMP of copper films because it is passive, nondestructive, and does not require light, which can have adverse effects on copper, to come into contact with the copper film. In addition, in contrast with optical methods that detect the state of film removal on only a single localized area, the method of the present invention monitors the state of film removal over the entire surface of the wafer.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.